1. Field of the Invention
This invention relates generally to high temperature electric tube furnaces. More particularly, it relates to such furnaces in which the tubular heating element is rendered warp-resistant by a compressible metal attachment between the tube and at least one of its supporting electric terminals.
2. Description of the Prior Art
The electric tube furnace is well known. Basically it is a thin tubular metal heating element, which is firmly attached at its ends to electrodes and which is heated by its resistance to a high amperage current. Heating generally is concentrated in the tube by the use of electrodes that are massive relative to the tube and offer less resistance to the flow of electricity. The material to be heated simply is inserted into an open end of the tube. Such furnaces have been used for a wide variety of purposes, including the maintenance of specimens of materials at predetermined temperatures during measurement of their thermophysical properties.
One such property is thermal diffusivity, which can be measured by the flash method. Typically, this measurement involves bringing a small, disc-shape specimen to a uniform temperature by centering it within the tube of an electric tube furnace, subjecting the front face of the specimen to a short energy pulse from a solid state laser or flash lamp and measuring the resultant temperature rise on the rear face with a thermocouple or optical pyrometer. The diffusivity is calculated from the specimen thickness and the time required for the rear face temperature to reach a specific percentage of its maximum value. Since the energy pulse is on the order of one millisecond or less, the temperature rise on the rear face usually is well below 5.degree. C. above the original specimen temperature and it is essential that one avoid distortions resulting from pre-energy pulse differences between the specimen and ambient temperatures. Ambient temperature control, and maintenance of that temperature uniformly over the specimen, is particularly critical at very high temperatures, such as those which often are required to measure thermal diffusivity near the phase transition temperature of the specimen material. Such near-phase transition temperatures often are in excess of 1000.degree. C. and may be as high as 3000.degree. C. or higher.
When the thermal diffusivity specimen and the tube of the electric tube furnace are resistant to oxidation and the maximum temperature does not exceed about 800.degree. C. to 1000.degree. C., both specimen heating and diffusivity measurement often may be done in the atmosphere. While bringing the tube up to the desired temperature for such atmospheric measurement, it is possible occasionally to reposition the electrodes which are firmly attached to the tube, so as to relieve the compressive force on the tube resulting from its linear thermal expansion. At higher temperatures oxidation of the specimen and furnace must be avoided by enclosing them in an oxygen poor zone, such as under an evacuated bell jar.
The presence of such jar makes it nearly impossible to manipulate its contents. Thus, as heating proceeds, the electrodes cannot be repositioned to accomodate the linear thermal expansion of the tubular heating element, and warping of the tube may occur. This effect becomes increasingly severe as the temperature is increased, and at temperatures from about 1600.degree. C. to about 3000.degree. C. or higher, non-uniform heating of the specimen (with accompanying erratic thermal diffusivity measurement,) may result from the fact that the walls of the warped tube are not equidistant from the specimen. In extreme cases, permanent damage may be done to the fragile tube. Similar problems often are experienced with prior art tube furnaces which are used for a wide variety of other purposes.